The invention relates to an infrared radiator configured as a surface radiator with a radiating body which, at its rear side, is heated by a burning fluid-air mixture and whose front surface emits the infrared radiation.
Infrared radiators configured as surface radiators are used in known manner in dryer systems for the drying of web shaped materials, for example, paper webs or cardboard webs. Depending upon the width of the web to be dried and the desired heating power, the requisite number of radiators with flush emitting surfaces are assembled into a drying unit.
In the publication xe2x80x9cRadiant efficiency and performance considerations of commercially manufactured gas radiant burners (Speyer et al., Exp. Heat Trans, 9, 213-245, 1996), various types of gas heated infrared radiators are compared with one another. A radiator is proposed which, among others, has a ceramic plate provided with holes through which a gas/air mixture flows and which burns on its surface. To avoid a migration of the flame and to increase the radiation efficiency, a metal grid is arranged ahead of the ceramic plate.
This known principle, which is used by many manufacturers, has the drawback that the radiation efficiency is comparatively small because of the low emission coefficient of the ceramic plate at high temperatures. In addition, the metal grid has only a limited life when the radiator is operated at high powers.
The object of the invention is to provide an infrared radiator configured as a surface radiator which has a high efficiency at temperatures above 1100xc2x0 C. and a long operating life.
This object is achieved with an infrared radiator configured as a surface radiator with a radiating body (15) which is heated at its rear side by a burning liquid/air mixture and from its front surface emits the infrared radiation. According to the invention the radiating body includes a multiplicity of throughgoing passages functioning as hollow space irradiators, in which the wall area/cross sectional area ratio in the flame-free region is greater than 10, preferably greater than or equal to 20.
Advantageously the passages are of circular cross section or are configured in the form of regular polygons whereby the length/maximum diameter ratio in the flame-free region is greater than 3, preferably greater than or equal to 5.
The radiating body can be constructed from a row of plates arranged in a spaced relationship to one another, whose intervening spaces form the passages, whereby the height of the plate/spacing between neighboring plates form a ratio in the flame-free region which is greater than 3, preferably greater than or equal to 5.
The proportion of the opening area of the passages to the total area of the front side of the radiating body amounts to at least 30%, preferably more than 50%.
The radiating body is preferably fabricated from ceramic.
The passages can have a depth less than 300 mm, preferably between 10 mm and 100 mm.
Advantageously the passages have a cross section widening toward the front side.
A burner plate can be spaced from the radiating body to form a combustion chamber therewith.
The radiating body can be made from a silicon carbide reinforced with carbon fibers.
The infrared body is preferably used for drying of web-shaped materials, especially paper webs or cardboard webs.
The invention makes use of the physical effect that a channel forming hollow radiator has at its opening an emission factor which increases with its ratio of wall area/cross sectional area. With a wall area/cross sectional area ratio greater than or equal to 20, a channel shaped hollow chamber radiator can have an emission factor of approximately 1 when it is fabricated from a ceramic with an emission factor of about 0.5.